This invention relates to a continuous process for the purification of glycerol water (sweet water) accumulating in the high-pressure hydrolysis of natural fats and oils with steam into fatty acid and glycerol.
For the production of glycerol, natural oils and fats are hydrolyzed with water under pressures of up to 100 bar and at temperatures of 100.degree. to 250.degree. C. Two phases are obtained, namely a light phase which contains the fatty acids and a heavy phase which consists essentially of glycerol and water and which is called "sweet water".
The need for high-purity glycerol requires removal of the impurities present in the natural fats and oils and in the sweet water. These impurities are naturally present in the renewable fats and oils while other impurities are added at the recovery stage (harvest, slaughterhouse). Accordingly, hydrolysis is preceded by purification of the oils and fats in a complicated process.
After the high pressure hydrolysis of the oils and fats into glycerol and fatty acids, a mixture of two phases is obtained, namely a heavy glycerol/water phase and a light fatty acid phase which is removed from the mixture by gravity separation. However, small quantities of fatty acids and other fats cannot be prevented from being removed with the glycerol/water phase. The small quantities of secondary products which are formed during the hydrolysis, for example diglycerides and monoglycerides, and triglycerides are also present in the sweet water. The impurities in the glycerol water are collectively referred to as matter of organic non-glycerol (MONG).
After phase separation, the glycerol water phase is restored to normal pressure, the organic impurities being highly emulsified in the process. The emulsion is concentrated by evaporation to 75-85% glycerol water and is then worked up into pure glycerol. The impurities lead during concentration to the formation of crusts on the heat exchanger surfaces and hence to considerable losses of performance. The reductions in capacity during concentration caused by the relatively long residence times, the product losses and the need for further purification steps are further disadvantages of the prior art.
A review of known processes for the production of pure glycerol from glycerol water (sweet water) can be found in J. Am. Oil Chemists Soc., 1979, pages 812 A-819 A.
The purification of glycerol water is problematical above all in two respects. Firstly, large product streams in excess of 20 m.sup.3 /h have to be continuously processed. Secondly, the product used for the hydrolysis of fats frequently changes in industrial-scale processes. Thus, palm oil, palm kernel oil, beef tallow, rapeseed oil, fish oil and other oils and fats are processed. The impurities in the glycerol water reflect the C-chain distribution of the starting products so that, for example, filtration cannot be carried out continuously on account of the different solidification points of the emulsified MONG phase.
Accordingly, the problem addressed by the present invention was to provide a particularly economic process of the type mentioned at the beginning which would be capable of purifying large product streams of glycerol water continuously, effectively and economically despite frequent changes of provenance.